The present invention relates to an image forming method and an image forming apparatus employed in electrophotographic copiers, printers, and the like.
In recent years, widely employed as electrophotographic photoreceptors, have been organic photoreceptors. Compared to other photoreceptors, organic photoreceptors exhibit advantages in that it is easy to develop materials which correspond to various types of exposure light sources ranging from visible light to infrared rays; it is possible to select materials which result in minimal environmental pollution; their production cost is lower, and the like. However, said organic photoreceptors exhibit disadvantages in that the mechanical strength is insufficient, and when producing numerous prints, the photoreceptor surface tends to be degraded or abraded.
Since electrical and mechanical external force is directly applied to the surface of electrophotographic receptors (hereinafter referred occasionally to as photoreceptors) upon employing a charging unit, development unit, a transfer means, a cleaning unit, and the like, durability is required to counter such force.
Specifically required is durability to resist wear and abrasion of the photoreceptor surface due to friction, surface degradation due to active oxygen such as ozone, nitrogen oxides, and the like, which are generated during corona charging.
Heretofore, in order to enhance the durability of organic photoreceptors, it has been highly demanded to minimize wear due to scrubbing of cleaning blades and the like. As an approach to meet said demand, techniques in which a protective layer of high strength is provided on the surface of a photoreceptor are investigated. For example, Japanese Patent Publication Open to Public Inspection Nos. 9-190004 and 10-251277 describe the formation of hardened resins, employing organic silicon modified positive hole transportable compounds.
In addition, as a method to minimize a decrease in the layer thickness due to said mechanical wear, the inventors of the present invention proposed a charge transportable polysiloxane hardenable resin layer as the protective layer of the photoreceptor (Japanese Patent Application No. 11-70308). Photoreceptors having said charge transportable polysiloxane hardened resin layer as the protective layer exhibit excellent characteristics in terms of strength. The protective layer prepared by employing said siloxane resins exhibits high strength as well as excellent wear resistance. However, since said siloxane resins are relatively hydrophilic, various problems occur during repeated image forming processes.
According to the investigation performed by the inventors of the present invention, hydrolyzing groups as well as silanol groups, which have not undergone reaction, tend to remain on the surface of the film formed by crosslinked organic silicon based siloxane resins. Thus problems occur in which said film tends to be affected by the adsorption of water molecules, under an atmosphere of high humidity. When there are many groups which have not undergone reaction, under an atmosphere of high humidity, water molecules as well as discharge products, which are formed during discharging, tend to be adsorbed. As a result, the surface resisitivity decreases, and problems such as image smearing (occasionally called image blurring), and the like, occur.
Further, in the course of the investigation performed by the inventors of the present invention, it has been discovered that phenomena such as the formation of said image smearing, and the like, are adversely affected by types of toners employed for the image formation. Namely, when a toner which exhibits large adhesion force to a photoreceptor, for example, a toner having a wide size distribution, is employed, the toner on some of the surface parts of the photoreceptor is not removed and repeatedly passes through the cleaning section. As a result, filming occurs on the photoreceptor surface, while the toner is subjected to effects such as pressing pressure of said cleaning section. In said filming forming parts, image problems such as the image smearing, black spots, and the like, tend to occur. It has been discovered that even when only slight filming forms on the photoreceptor comprised of said siloxane resins, these image problems tend to occur. Further, the photoreceptor surface, on which said filming has once been formed, is subjected to non-uniform wear due to the cleaning member. As a result, problems occur in which cleaning properties degrade accumulatively due to said non-uniform wear.
An object of the present invention is to provide an image forming method and an image forming apparatus which result in high durability as well as high image quality, and more specifically to provide an image forming method and an image forming apparatus comprising an organic electrophotographic photoreceptor comprised of siloxane based resins, which result in high durability and consistently form excellent images under severe ambient conditions such as high temperature, high humidity, and the like.
The object of the present invention is accomplished employing any of the embodiments described below.
1. An electrophotographic image forming method including electrostatic charging on a photoreceptor, imagewise exposing on the photoreceptor to form a latent image, developing the latent image by a developer containing a toner, transferring the developed toner onto an image forming material, and cleaning residual toner on the photoreceptor, wherein decrease xcex94Hd (in xcexcm), in layer thickness per rotation in wear test of the organic electrophotographic photoreceptor defined below, is in the range of 0xe2x89xa6xcex94Hd less than 1xc3x9710xe2x88x925, and a toner used in said development process is one which is obtained by suspension-polymerizing a polymerizable composition consisting of at least a polymerizable monomer and a colorant in a water based medium or one which is obtained by fusing resinous particles in a water based medium.
An electrophotographic photoreceptor connected to a driving section was brought into contact with a cleaning blade having a hardness of 70xc2x11xc2x0, an impact resilience of 35xc2x11 percent, a thickness of 2xc2x10.2 mm, and a free length of 9xc2x10.1 mm under conditions of a contact angle of 10xc2x10.5 degrees in the counter direction and a thrust amount of 1.5xc2x10.2 mm. While rotating said electrophotographic photoreceptor employing said driving section so that one rotation is completed within 0.1 to 10 seconds, toner particles having a volume average particle diameter of 8.5xc2x10.5 xcexcm, which was blended with powder having a number average particle diameter of 10 to 40 nm as the external additive, in an amount of 1xc2x10.1 percent by weight with respect to said toner, was subjected to development so as to result in an adhered amount of 0.15xc2x10.05 mg/cm2. After development, said toner particles were removed. When said electrophotographic photoreceptor was subjected to at least 100,000 rotations, the layer thickness variation amount of said photoreceptor was measured and the value obtained by dividing the resultant amount by the number of rotations was designated as the layer thickness decreasing amount per rotation.
2. The image forming method of item 1 wherein the toner used in said development process is one which is obtained by suspension-polymerizing a polymerizable composition consisting of at least a polymerizable monomer and a colorant in a water based medium.
3. The image forming method of item 1 wherein the toner used in said development process is one which is obtained by fusing resinous particles in a water based medium.
4. The image forming method of item 1, wherein the organic electrophotographic photoreceptor comprises an overcoat layer comprising a siloxane based resin layer containing structural unit having charge transferability and a bridge structure.
5. The image forming method item 4, wherein said siloxane based resin layer is one which is formed by allowing an organic silicon compound having a hydroxyl group or a hydrolyzing group to react with the compound represented by the aforementioned General Formula (1).
Bxe2x80x94(R1xe2x80x94ZH)m
wherein B represents a univalent or multivalent group comprising structural units having charge transportability, R1 represents a single bond or divalent alkylene group, Z represents an oxygen atom, a sulfur atom or NH, and m represents an integer of 1 to 4.
6. The image forming method of item 5, wherein Z in the aforementioned General Formula (1) is a oxygen atom.
7. The image forming method of item 1, wherein the siloxane based resin layer contains an antioxidant.
8. The image forming method of item 7, wherein said antioxidant is a hindered phenol based antioxidant or a hindered amine based antioxidant.
9. The image forming method of item 4, wherein the siloxane based resin layer contains organic or inorganic fine particles.
10. The image forming method of item 4, wherein the siloxane based resin layer contains colloidal silica.
11. The image forming method of item 4, wherein a toner employed in said development process has a number average particle diameter of 3 to 8 xcexcm.
12. The image forming method of item 4, wherein ratio of toner particles having no corners is at least 50 percent by number, and the number variation coefficient in the number particle size distribution is no more than 27 percent in the toner.
13. The image forming method of item 1, wherein ratio of said toner particles, in the range of a shape coefficient of from 1.0 to 1.6, is at least 65 percent by number of toner particles.
14. The image forming method described of item 1, wherein in a particle number based histogram showing a particle size distribution, in which, when the diameter of toner particles is represented by D (in xcexcm), natural logarithm of D, i.e. lnD is taken as the abscissa and said abscissa is divided into a plurality of groups at an interval of 0.23, sum M of relative frequency m1 of toner particles included in the most frequent group, and relative frequency m2 of toner particles included in the second most frequent group, is at least 70 percent.
15. An image forming apparatus comprising an organic electrophotographic photoreceptor, a charging unit, an exposing unit, a development unit, a transfer unit, and cleaning unit, wherein decrease xcex94Hd (in xcexcm), in layer thickness per rotation in said wear test of said organic electrophotographic photoreceptor, is in the range of 0xe2x89xa6xcex94Hd less than 1xc3x9710xe2x88x925, and a toner used in said development process is one which is obtained by suspension-polymerizing a polymerizable composition consisting of at least a polymerizable monomer and a colorant in a water based medium or one which is obtained by fusing resinous particles in a water based medium.